1. Field
The technology described herein relates to alternating current (AC) to direct current (DC) power supplies, and more particularly to power line current fed power supplies producing stable load currents and methods relating to the same.
2. Related Art
The operation of utility power lines can be monitored with sensors. Utility power lines typically undergo changes in operation, including expected and unexpected changes. Expected changes include changes in the amount of current on the utility power line due to changes in user demand. Unexpected changes include changes in the amount of current on the utility power line due to fault conditions. Sensors can be used to monitor the described changes on the utility power line, and therefore provide information useful in assessing operation of the utility power line.
Sensors used to monitor the operation of utility power lines are typically powered in one of two manners, presuming the sensors operate on direct current (DC) power. One manner of powering such sensors is with a battery. A second manner of powering such sensors is with an alternating current (AC) to direct current (DC) power supply which uses the AC current of the utility power line being monitored to produce a DC output signal for powering the sensor.
FIG. 13 is a circuit schematic of a conventional AC to DC power supply switching circuit providing a voltage output. The power supply includes a front end current transformer CT1, which simultaneously measures power line current and supplies current to power electronic circuits. The current transformer CT1 has a split magnetic core and operates in a linear mode. The current transformer CT1 is built with a single turn primary winding utilizing an AC power line wire or a cable, which is an ideal AC current source with variable line current IL. The current transformer CT1 also has a secondary winding W.
Multiple components are connected to the secondary winding W of the current transformer CT1. A ballast capacitor C1 is connected across the secondary winding W and reduces the output current of the current transformer. A rectifier bridge having diodes D1, D2, D3 and D4 is also coupled to the secondary winding W of the current transformer CT1.
The power supply in FIG. 13 also includes an on/off parallel switching voltage regulator to absorb extra current after the rectifier bridge. The voltage regulator includes a transistor switch SW connected in parallel to the output of the diode bridge, a comparator U1 controlling transistor switch SW, a series diode D5 for reverse current protection, a Zener diode Z1 for over voltage protection, an output energy storage capacitor C2, and a voltage divider with resistors R1 and R2.
Transistor switch SW is provided with a switching MOSFET M1 with gate resistors R3 and R4 and is controlled directly by the comparator U1 via a series gate resistor R3 as the switching frequency is low. When the output of the voltage divider formed by R1 and R2 is higher than the reference voltage, the comparator turns the switch SW “on,” diverting rectified current from the output of the power supply. When the output voltage Vout becomes too low, the switch turns “off,” restoring current flow for charging capacitor C2.
To reduce current in the switch SW the transformer CT1 is provided with an increased number of turns in the secondary winding W, which results in the output voltage Vout being higher (about 23V) than many electronic blocks require. Thus, the power supply of FIG. 13 is conventionally provided with a step down series switching regulator (not shown) that steps down the output voltage Vout from about 23V to 3.3V.